The present invention relates to an air-fuel ratio controlling apparatus for an internal-combustion engine, and in particular it relates to an apparatus for estimating an air-fuel ratio of each cylinder using an internal cylinder pressure sensor to make air-fuel ratio of each cylinder substantially the same.
Sometimes, errors are produced in the amount of intake air into multiple cylinders of an engine with respect to a desired amount due to aging of an air intake system and/or parts of a valve-actuating system and others. Such error differs for each cylinder because the error depends on mechanical factors. A command value for a fuel injection amount to be transmitted to each cylinder is the same for all cylinders because a control is carried out such that air-fuel ratios for all cylinders are the same. Thus, although each cylinder receives the same control command value, unevenness of air-fuel ratio is produced among the plural cylinders.
When air-fuel ratio unevenness is produced among the cylinders, a catalyst purification rate may decrease resulting in poor emission performance. When the unevenness of the air-fuel ratio becomes excessively large, misfiring may take place in the cylinder that is in an excessively lean or rich state. Even when the misfiring does not take place in such a state, drivability will deteriorate as idling vibration, surging or the like may be caused when a significant stepwise torque difference is produced among the cylinders.
The Japanese Patent Application Publication No. H2-99745 discloses a technique comprising detecting a crank angle when internal cylinder pressure reaches maximum as detected with a pressure sensor disposed in each cylinder and estimating an air-fuel ratio of each cylinder based on the crank angle at the time of ignition, thereby controlling an air-fuel ratio of each cylinder. An actual air-fuel ratio is controlled to match a desired air-fuel ratio based on a correlation between variation of an air-fuel ratio in a cylinder and a combustion time.
However, the duration from ignition to firing of air-fuel mixture and the duration from start of firing of air-fuel mixture to the time when the internal pressure reaches a maximum vary depending on fuel characteristics (volatility) and/or an internal temperature of the cylinder. For this reason, if the air-fuel ratio is estimated based on the duration from ignition to the time the internal pressure reaches the maximum, precision of the estimation would be poor, leading to a wrong air-fuel ratio control.
Accordingly, it is an objective of the present invention to provide an apparatus for performing an air-fuel ratio control with a good precision.